Psilochasmus oxyurus, type species of the genus Psilochasmus Lühe, 1909, has been redescribed from whole mounts and transverse, horizontal and sagittal sections. Twenty-five worms from one common mallard, Anas platyrhynchos L. and 12 from one greater scaup, Nyroca marila L. are considered as conspecific with P. oxyurus, although they show some variations in body size and shape of the testes. Of the other eight known species of Psilochasmus, five have already been synonymized and the remaining three, namely, P. indicus Gupta, 1957, P. alii Jaiswal, 1957, and P. megacetabulus Jaiswal, 1957, are declared synonyms of P. oxyurus.

An experiment has been conducted to examine the effects of discontinuous infection with H. contortus in sheep on the development of resistance to infection. When sheep were given four doses of 3000 larvae at fortnightly intervals and these sensitizing infections were removed on day 56, there was no evidence of resistance at that time nor of any residual effect of the sensitizing infections on the response to challenge at day 140. When, however, the sensitizing infections were permitted to persist until day 132, there was strong resistance to the establishment of a challenge infection whether or not the sensitizing worm burden was removed prior to challenge, and, in the case of superimposed challenge, resistance was coupled with rejection of most of the sensitizing population. When a single infection with 3000 larvae was followed 22 days later with a further single dose of 3000 larvae, there was no evidence of resistance whether or not the first infection was removed with anthelmintic. If the interval between two doses of 3000 larvae was extended to 76 days there was resistance to the establishment of the second dose if this was superimposed on the first, but not if the first infection was removed 8 days prior to the second dose.

It is suggested that the manifestations of resistance appear in a sequence of (i) resistance only to a superimposed infection, followed by (ii) resistance to a challenge infection in the presence or absence of a previously existing worm burden, coupled in the former case with rejection of much of the existing population. This sequence could be related crudely to a product of larval dose and time. Although larval dose and time were to some extent confounded in the design of the experiment strong resistance was associated with prolonged uninterrupted infection.

Regional differences in the tegumentary tissue of Moniezia expansa, as revealed at the light-microscope level by histological and histochemical techniques, are described and evidence for secretory activity by the interproglottidal glands is presented.

In very immature proglottides the interproglottidal glands are at the ‘precryptic’ stage. Gland cells may be differentiated from other tegumentary cells by their high RNA content and in certain gland cells the presence of an alcian blue staining material.

In mature proglottides the glands consist of rosette-like clusters of cells around crypt-like intuckings of the tegument. Two types of cells are found in the gland, small alcian blue-staining cells which are most numerous in the neck region of the crypt, and larger cells, the predominant gland cells, which do not stain with alcian blue but possess non-specific esterase activity. No other tegumentary cells in Moniezia exhibit this activity. Esterase and phosphatase activity is found in the tegument and crypt of the glands and in the interproglottidal folds.

The non-enzyme histochemistry confirms and extends the observations of previous workers.

Cytochrome oxidase and succinic dehydrogenase were detected in the tegumentary cells and tegument. Very strong reactions were given in the neck and scolex, with a progressive diminution of activity posteriorly along the strobila. Very low activities were recorded in the tegument of the glands.

Of 414 Bermuda fishes belonging to 58 species 69 belonging to 20 species were infested with adult or larval cestodes. Five species of adult and five species of larval cestodes were recovered. There are 18 new host records and seven new records for Bermuda. Four of the adult cestodes were found in the cub shark Carcharinus platyodon (Poey). Larval cestodes were found in 66 individuals of 18 species of fishes mainly groupers and rock fishes from the outer reefs and deep water where sharks abound. It is suggested that the restriction of cestodes of Bermuda fishes is due to the isolation of the fishes into the deep water population of the outer reefs and beyond and the shallow water population of the inner reefs.

Acompsocephalum tortum (Linton) was the only cestode from a teleost Synodus intermedius (Agassiz). It has been included in a new family in the Pseudophyllidea but has affinities, also, with the Spathebothridea. It is described and its significance relative to the evolution of the strobilate cestodes is discussed.

The immunogenicity of larval Trichuris muris in Schofield strain albino mice has been studied using infections abbreviated by chemotherapy. When mice were exposed to an initial infection lasting from 7 to 14 days their immunity to reinfection increased progressively, but the immunity stimulated by abbreviated infections was less than that stimulated by uninterrupted immunizing infections which last for approximately 16–18 days. The immune response of mice to T. muris appears to be stimulated by, and effective against, larvae of all ages and does not show stage specificity.

Three new species of dactylogyrid monogeneans with three anchors are described: two of them, Eutrianchoratus magnum and E. minutus, from Ophiocephalus obscurus, and Heteronchocleidus ctenopomae from the anabantid Ctenopoma kingsleyae, all from south Ghana.

Analysis of the variation in the morphological patterns of the anchors and the bars in the known species of the Heteronchocleidinae suggests gradual specialization from an ancyrocephaliid type with two pairs of anchors to types with three anchors arranged in triangular symmetry.

Heteronchocleidinae occur on both ophiocephaliid and anabantiid fish in Southeast Asia as well as in West Africa.

Changes associated with schizogony and gametogony in Eimeria tenella have been studied by means of electron microscopy.

In infections of E. tenella both small and large schizonts are formed, and in both cases merozoites are developed by a surface ‘budding’ mechanism. The final severance of the merozoite from the schizont appears to give rise to the dorsal pore of the merozoite. Newly formed merozoites contain structures which are absent in the mature organism.

Early in microgametogony the nucleus of the microgametocyte divides repeatedly to produce a number of smaller nuclei which migrate peripherally in the cell. The chromatin aggregates into the peripheral part of each nucleus, which then elongates to form the nucleus of the of microgamete. That part of the gametocyte nucleus which is devoid of chromatin, probably gives rise to the perforatorium and three flagella of the gamete. Residual material is left in the gametocyte cytoplasm in the form of a gametogenic cyst.

‘Wall-forming’ bodies are present in the earliest macrogametocyte and ‘dark bodies’ are developed later. After fertilization these bodies migrate to the periphery of the zygote. In E. tenella the oocyst wall is trilaminate. The limiting membrane of the zygote separates from the cell to become the outermost membrane of the cyst wall. The middle layer of the cyst wall is developed from the ‘dark bodies’, and the ‘wall forming’ bodies give rise to the inner layer of the oocyst wall.

Two experiments are described in each of which three groups of calves received infective larvae of Ostertagia ostertagi daily, each group at a different rate. Changes in worm burdens were followed by the periodic slaughter of calves.

Numbers of adult worms quickly rose to and remained at levels which bore a positive relation to the infection rate. There was evidence that a turnover of worms was occurring and it appeared that burdens of adult worms were regulated by a loss of worms which depended on the number present. As the host's experience of infection increased, the adult worms present were smaller and an increasing proportion of females lacked a fully developed vulval flap. Evidence is presented which suggests that these were effects of resistance and that they depended on separate mechanisms.

Faecal egg counts of groups carrying different numbers of worms rose to the same peak and then declined according to the same logarithmic curve. This finding, together with data concerning the number of eggs in the uteri of the worms, suggested that the egg output of the entire population was restricted by the host to a limit which depended on previous egg-laying.

Infection did not have an adverse effect on the growth of the calves unless the infection rate exceeded a critical value. The resistance of calves to infection was impaired by clinical ostertagiasis.

In two experiments in which two different strains of T. annulata were used it was possible to induce theileriasis by injection of the crushed infected unfed H. dromedarii. The nature of the disease corresponded with that transmitted naturally by the feeding of the tick vector.

Immunization was performed against Schistosoma mansoni or S. japonicum infection in rhesus monkeys either with living schistosomula and young adult worms or with only schistosomula by intra-peritoneal injection. Each monkey was immunized four or more times and subsequently challenged. A significant degree of resistance against the challenge was produced. The injected worms lived in the peritoneal cavity for a short period, then died and disintegrated therein, thus obviating their inhabiting the circulatory system. The functional antigens were evidently derived from the metabolic products of the immunizing living schistosomes in the peritoneal cavity.

The results of the present experiment indicate that the main immune mechanism induced by the intraperitoneal injection of living schistosomula and young adult worms is that which inhibits the development of the late stage of schistosomula and the early stage of adults which have originated from the challenge infection. This immune mechanism may be induced by the functional antigens either from both the schistosomula and adult worms or from the schistosomula alone.

Single receptors seen with the electron microscope to consist of a terminal cilium embedded in a nerve bulb have been found in the skin parasitic monogenean Gyrodactylus sp., adult and larval Entobdella soleae, Leptocotyle minor (adult) and in the endoparasitic juvenile of Amphibdella flaviolineata. Their distribution in Gyrodactylus sp. has been mapped using phase-contrast microscopy, and staining with the indoxyl acetate method for non-specific esterases revealed a nervous connexion between the lateral ventral nerve cord and the ‘tangoreceptor’. The general relationships of the nervous system in Gyrodactylus were investigated by staining with the thiocholine method for cholinesterase. The ‘sensory’ neurone contains vesicles, microtubules and mitochondria and the whole nerve bulb is sealed into the epidermis by means of septate desmosomes. The basal body of the cilium is not greatly modified and there is no obvious rootlet system. The terminal cilium of single receptors in Entobdella soleae has a 9 + 2 structure. A compound sense organ from the head of Entobdella soleae has also been described. The significance of the fibre arrangement in the terminal cilia and the possible roles of these sense organs have been discussed.

I should like to thank Dr D. L. Lee for training in electron microscope techniques and for his encouragement, Dr P. Tate for his continued interest and the staff of the Marine Biological Laboratory, Plymouth, especially Mr G. Best and Mr J. E. Green, for their assistance. This work was conducted during tenure of a Science Research Council Fellowship and the Tucker-Price Research Fellowship of Girton College.

The fine structure of two kinds of compound presumed sense organs from the heads of three skin parasitic monogeneans Gyrodactylus sp. Entobdella soleae (larva only) and Acanthocotyle elegans is described. One kind of compound receptor consists of a number of associated sensilla, each ending in a single cilium (the spike sensilla of Gyrodactylus and the cone sensilla of E. soleae oncomiracidium).

The other kind of compound organ is made up of one or a few neurones only, each of which bears many cilia (pit organs of E. soleae oncomiracidium and feeding organ sensilla of Acanthocotyle elegans). The spike sensilla of Gyrodactylus have also been studied using a Cambridge Instrument Co. Stereoscan electron microscope and by phase-contrast microscopy. The ciliary endings of all these sense organs are highly modified and have lost the 9 + 2 structure, being packed with many fibres. The fibre arrangement in the cilia of the cone sensillae of E. soleae oncomiracidium and the feeding organ sensilla of A. elegans has been compared with that in the ciliary endings of other invertebrate mechano- and chemoreceptors. The possibility that the spike sensilla of Gyrodactylus may be chemoreceptors has been discussed but it is considered premature to attempt to assign functions to the other sense organs studied. Electron dense membrane-bound inclusions occurring specifically in the nerves supplying the spike sensilla of Gyrodactylus may be neurosecretory.

In the system investigated the spleen was essential in most rats for the acquisition of immunity and its maintenance in the presence of the piroplasm.

It is stressed that model immunological systems involving the role of the spleen do not simulate the complexity of a parasitic infection. The roles of the spleen as an antibody producer, and as a site of phagocytosis and haemopoiesis are discussed in relation to the control of Babesia rodhaini by the rat. The possibility that the piroplasm may be able to undergo antigenic variation is considered.

Most epidemiological models assume that disease is the inevitable outcome of infection (see Bailey, 1957). Yet as Dubos (1965) has said; ‘Throughout nature, infection without disease is the rule rather than the exception’. There are, in fact, many diseases whose distribution cannot be explained solely by a consideration of the probabilities of host parasite encounters. In these cases, a diseased state is only one possible outcome of an interaction between parasite phenotypes, host phenotypes and the external environment. Haemonchosis is an example of such a disease and has been studied extensively in quantitative terms (see Whitlock & Georgi, 1968).

1. Three new species of eugregarine are described from the mid-gut of the larva of Costelytra zealandica (White).

2. Euspora zealandica occurs in the anterior region of the mid-gut near the gut caeca. Slictospora costelytrae occurs just posteriorly to E. zealandica. Euspora sp. occurs posteriorly to the malpighian tubules, but was found only rarely.

3. A description of the species and the life-cycle of two species are given.

4. The life-cycle of S. costelytrae takes about 8 weeks and that of E. zealandica 6 weeks.

5. Gametocysts will develop only at a humidity of 95–100 %.

6. The bi-associative species, E. zealandica, is more abundant in the 1st and 2nd instars than the large species, S. costelytrae. Both species are equally abundant in the 3rd instars, but the peak for incidence was in May for E. zealandica and June for S. costelytrae. This is related to the longer period of time taken for the life-history of S. costelytrae.

7. The incidence of gregarines builds up in each instar to over 90% then falls off, the time of fall off corresponding with the production of gametocysts which pass out with the faeces of the grub.

8. The gregarines mostly complete their cycle before the instars moult. This is borne out by the observation that cysts were not found in the smallest size group of each instar. There were many cases in the larger size group of all three instars where cysts only occurred.

9. Gametocysts are produced mainly in the autumn.

10. Gregarines were not found in grubs approaching metamorphosis (prepupae) nor in pupae nor adults.

11. It seems unlikely from the results that the gregarines have an adverse effect on the grubs as, in general, it was found that the larger grubs had the greater number of gregarines present, but further work on this is needed.

12. The life-history of the gregarines is closely correlated with the life-history of the beetle.

13. The presence of the gregarines only in the larval stages is related to the different habitat and behaviour of larva and adult.

I am grateful to the University Grants Committee for financial support.

1. Procercoids of Schistocephalus solidus reared in copepods were found to infect marine and freshwater varieties of Gasterosteus aculeatus, and Pungitius pungitius, but not Barbus ‘Schuberti’.

2. In primary infections growth of S. solidus plerocercoids in P. pungitius was much slower than in G. aculeatus and ceased after 6–8 days; plerocercoids were dead by day 11 at 19·5 °C, and by day 14 at 10 °C.

3. A challenge infection, following two previous infections given 14 and 28 days before, becomes established in G. aculeatus, but in P. pungitius plerocercoids are rejected within 3–5 days.

4. The possible rejection mechanism involved in P. pungitius to primary and tertiary infections is discussed and electronmicrographic evidence presented of a degenerative change in the tegument visible by 8 days in a primary infection. Prior to 8 days the survival and growth of S. solidus plerocercoids surgically transferred to G. aculeatus, indicated no irreversible damage had occurred.

Of two groups of calves, one was regularly treated with the cortisone-derivative, β-methazone. This treatment greatly reduced the ability of the calves to produce antibody and gave rise to a number of side effects. Calves of both groups were given 1500 infective larvae of Ostertagia ostertagi daily. A milder régime of cortisone administrations using prednisolone was employed in a second experiment in which two comparisons were made. Worm burdens of calves infected with 1500 larvae daily and receiving cortisone were compared with those of control calves infected at the same rate and cortisone-treated calves receiving 1500 larvae per day were compared with cortisone-treated calves infected at twice this rate.

By periodic post-mortem examination it was shown that even in cortisone-treated animals the worm burden was regulated at a level only slightly higher than in control calves, apparently by a density-dependant loss of worms. A more rapid loss of worms which occurred for a period in the control calves was prevented in cortisone-treated animals unless the infection rate was high.

The limit set by the normal host to the total egg output of its worm population was raised by cortisone treatment, very greatly in the first experiment, rather less in the second in which the cortisone treatment was less severe. The effect of host resistance on the length of worms and on the incidence of females without vulval flaps was greatly reduced by cortisone. The inhibition of development was not affected.

Various parasympathomimetic drugs were injected per anus into female R. appendiculatus, previously fed for 5 days on rabbits, in attempts to induce the ticks to salivate. Pilocarpine nitrate and arecoline hydrobromide gave the best results and these drugs were therefore injected into ticks of various weight ranges and at various concentrations of the drug and the diluent, saline. The drugs were most effective at a concentration of 10% in 6% sodium chloride solution. If ticks salivated, the amount of saliva they produced could be positively correlated with their weight.

Sections of salivary glands from 5-day-fed female ticks in the corresponding weight ranges were examined for infection with T. parva. There appeared to be an inverse relationship between the infection rate of the ticks, and the number of infected acini observed, with increase in their weight. It was established that the greatest numbers of infective particles of T. parva should be obtained by the injection of 1 μl of 10% pilocarpine nitrate in 6% sodium chloride solution into ticks weighing between 3.1 and 13.0 mg. Twenty-five infected ticks in this weight range were injected, and the resultant saliva added to concentrated autologous leucocytes in phosphate-buffered saline before injection into a steer. The animal underwent a typical East Coast fever reaction and died, thus indicating that infective particles had been harvested by our methods.

Acknowledgements are due to many colleagues for assistance in a variety of ways. Mr M. P. Cunningham, Mr C. G. D. Brown, Dr J. K. H. Wilde, Dr R. Tatchell, Mr M. L. Burdin and Dr G. H. Freeman have given us a great deal of helpful advice. Mr C. Boarer and Mrs D. Backhurst supervised and carried out the processing and staining of the tick sections. Mr B. Wood carried out the weighing of the ticks and prepared the photographs.

Prototransversotrema steeri gen.nov., sp.nov. is found under the scales of the mullet, Aldrichetta forsteri, a euryhaline fish.

It is distinguished from Transversotrema Witenberg, the only other genus of the family Transversotrematidae by the possession of an oral sucker as well as a pharynx. This necessitates alteration of the family diagnosis.

Knowledge of the life-histories of Transversotrema spp. is summarized. The so-called ‘metacercariae’ found under the scales of freshwater and euryhaline fish are regarded as adults. When they have no eggs or only one egg they have probably developed in a fish which is not the normal host.

P. steeri escapes from under the scales of the fish shortly after the latter is caught. It is suggested that this habit may account for the small number of species known in the family.

Two strains of Eimeria acervulina were compared. The more recently isolated strain had a higher reproductive potential and produced slightly longer oocysts. Otherwise both strains conformed to the characteristics of the species.

The two strains were not immunologically identical and birds which were solidly immune to one strain would support a degree of infection with the other.

Acknowledgements are due to Mr C. C. Norton for technical assistance and to Dr Helen Hein who provided access to experimental data prior to publication and discussed some aspects of the results. The statistical calculations were carried out by Miss N. Hebert.